Marathon swimmer Patti Bauernfeind, stung by hundreds of jellyfish, gave up her quest to complete a 23-mile swim of Monterey Bay after about 4½ hours Saturday, just short of the midway point of the quest.

Bauernfeind, 43, left Seabright Beach in Santa Cruz at midnight, hoping to conquer the currents, the cold water and the jellyfish to become the second swimmer ever to swim the bay. Cindy Cleveland did it in 1983, and five other swimmers have failed to equal the feat since then.

Bauernfeind, a Pleasanton resident, swam without a wet suit in accordance with English Channel open-water swimming rules.

“The toxins of the jellyfish stings just became too much for her,” said her coach, Joel Wilson of Santa Cruz Masters Aquatics. “She swam great for four hours — she was fast and strong — but then it was almost like jumping off a cliff for her. She tried to hang in there, but the stings were too much.” Bauernfeind, who completed a 22-mile swim of Lake Tahoe last August, was attempting the Monterey Bay swim as part of a charity effort to raise $50,000 to build a school in Afghanistan. She tutors Afghan students — former refugees — in the San Francisco Bay Area.

Another swimmer, Bruckner Chase of Santa Cruz, plans to attempt the Santa Cruz-to-Monterey swim Tuesday, for the second year in a row. Chase’s 2009 swim also was truncated by jellyfish stings.

He plans to enter the water at Seabright Beach at 4 a.m. and hopes to exit somewhere between 3 to 6 p.m. at San Carlos Beach in Monterey, a distance of about 25 miles.

Chase is making his swim to raise awareness about ocean conservation issues, including the Seafood Watch Program and the Blue Ocean Film Festival.